“It’s so flat, you can watch your dog run away for three days.”
– A popular joke about flat places everywhere
Saskatchewan. Nebraska. Iowa. Illinois. The joke has been said about almost every level, treeless region in North America. Why?
It’s natural to wonder why mountains are tall. The Rockies form a 3,000-mile-long wall from New Mexico to British Columbia. In Colorado alone there are 53 peaks above 14,000 feet. Mountains tower over us, expecting our awe and admiration.
Perhaps less instinctive is to ponder the level surface of the plains.
One can drive three hours across central Illinois and adjust the steering wheel only once or twice—with no topographic obstacles, Illinois’ interstates are like the sides of polygons, their vertices in cities such as Chicago, Peoria, Springfield, Decatur, and Champaign.
Why is Illinois so horizontal?
It’s the same reason Yosemite Valley in the Sierras is so vertical—glaciers. In mountainous areas, glaciers are careful sculptors of U-shaped valleys and coastal fjords. In Illinois, they acted more like bulldozers and dump trucks, burying every hill and dale with a level expanse of debris.
It was a massive operation.
From 2.6 million years ago to the present, a time span geologists refer to as the Quaternary Period, Earth’s history has been defined by ice. Hundreds of feet to several miles of ice covered up to 30% of the globe at times. Northern Europe, Siberia, Greenland, Iceland, Canada—as far south as 38°N in Illinois—were white year round.
Picture this—somewhere in Canada, it snows one winter, but summer never arrives. The snow doesn’t fully melt. The following winter, it snows again. Add more snow the winter after. Each year the snow piles up. At the bottom of the pile, the pressure is enormous. The buried snow recrystallizes, becoming ice.
Fast forward a few hundred years, and our mound of snow and ice is so heavy that it bulges out to the sides, flowing under its own weight. It is now a glacier. The glacier oozes outward, like spilled molasses on a counter top, to cover the continent.
At times this Canadian-born glacier reached almost all the way to the southern tip of Illinois.
If you were to pick up a handful of Illinois’ glacial till—the debris-like assortment of mud, silt, sand, and gravel dumped by glaciers on the state—you’d find rocks that are far away from home alongside established locals. Pebbles of gneiss and granite over a billion years old from the Canadian shield, over 500 miles away, occupy the same handful as bits of sandstone, limestone, and chert from Illinois’ own buried bedrock. The glaciers were indiscriminate in their hauls of abraded and plucked rock as they bulldozed their way south. For this reason, like the ethnic melting pot of Chicago, glacial till is a hodgepodge of origins.
The glaciers came in waves. Every hundred thousand years or so, during times of glacial advance, ice lapped at the shores of 40°N. Advances alternated with periods of retreat, when the glaciers withdrew to the far north of Canada. Why the back and forth? And why so cold in the first place?
The answer to both questions may seem abstruse: orbital mechanics. The sun is the ultimate driver of climate on Earth, controlling not only day-to-day weather patterns but also climate cycles on geologic time scales.
The Earth tilts and wobbles. It pulls away from the sun and then moves in close again, a carefully choreographed dance. Together, the movements are called Milankovitch cycles.
These cycles influence the intensity of seasons in one way or another. For example, when the Earth’s tilt is high, summers are hotter and winters are colder, but when the tilt is low, both summers and winters are milder.
However, don’t be tempted to blame this past winter’s weather on Milankovitch cycles. The eccentricity, obliquity, and precession cycles don’t come full circle for 400 thousand, 41 thousand, and 26 thousand years, respectively.
But if everything lines up just right—say, winters are cold, summers are mild and snow doesn’t completely melt—the Earth can experience one long, nasty, glacier-filled winter.
Technically, winter has lasted for 2.6 million years now. That’s 650 times longer than all of human history, and it’s still taking place.
Ice sheets live on in Greenland and Antarctica to remind us it is, in fact, still the ice age. But we don’t need to be polar explorers to observe glacial landscapes; evidence of our icy and recent geologic past is written all over the northern hemisphere, from the fjords of Norway to the fields of Illinois.
Most U.S. corn grows in the Midwest, in rich, wind-scattered soil from ancient glacial streams. Farmhouses and even entire towns are built on the superior drainage of glacial moraines, the debris piles left in the wake of a glacier’s retreat.
Glacial features even crop up in places that never saw a single glacier during the Quaternary. Golf courses everywhere are carefully constructed to mimic the glacial landscapes of northeastern Scotland, where the sport was born. Yes, even in Florida, the U.S. state with the greatest number of golf courses, we are reminded of our chilly position in Earth’s history.
So why is Illinois so flat? In short, the state’s striking horizontality reflects dramatic changes in Earth’s climate. Orbital dances, glacier advances, and a deep, even coating of glacial debris make Illinois the second flattest state in the U.S., where a slight rise in the road may speak for over two million years of Earth’s history.